1 Introduction
3The organic content of sludges and soluble wastes can be reduced by controlled bac-
terial activity. If the bacterial activity is anaerobic, the reduction in organic content
is achieved through sludge digestion. If the bacterial activity is aerobic, the reduc-
tion in organic content is achieved through sludge stabilization.
Anaerobic digesters having suspended bacterial growth are commonly used at
municipal wastewater treatment plants to degrade (digest) sludges (Figure 1.1). With
the development of anaerobic digesters having fixed-film bacterial growth (Figure
1.2), more and more industrial wastewater treatment plants are using anaerobic
digesters to degrade soluble organic wastes. Anaerobic digesters represent catabolic
(destructive) processes that occur in the absence of free molecular oxygen (O 2 ).
The goals of anaerobic digesters are to biologically destroy a significant portion
of the volatile solids in sludge and to minimize the putrescibility of sludge. The main
products of anaerobic digesters are biogas and innocuous digested sludge solids.
Biogas consists mostly of methane (CH 4 ) and carbon dioxide (CO 2 ).
Primary and secondary sludges are degraded in anaerobic digesters (Figure 1.3).
Primary sludge consists of the settled solids from primary clarifiers and any colloidal
wastes associated with the solids. Secondary sludge consists mostly of waste-
activated sludge or the humus from trickling filters. The mixture of primary and
secondary sludges contains 60% to 80% organic matter (dry weight) in the forms
of carbohydrates, fats, and proteins.
The mixture of primary and secondary sludges is an ideal medium for bacterial
growth. The sludges are rich in substrates (food) and nutrients and contain a large
number and diversity of bacteria required for anaerobic digestion.
The anaerobic digester is well known as a treatment process for sludges that
contain large amounts of solids (particulate and colloidal wastes). These solids
The Microbiology of Anaerobic Digesters, by Michael H. Gerardi
ISBN 0-471-20693-8 Copyright © 2003 by John Wiley & Sons, Inc.